In-Season Tools Manage N Closer To Margin
Pre-sidedress nitrate test (PSNT) and chlorophyll meters offer corn growers confidence tools.
Current nitrogen (N) fertilizer recommendations in Colorado are based on soil samples taken in the fall or in the early spring. However, most N uptake by corn occurs in midsummer from the 8-leaf growth stage to pollination. Mineralization of N from manure or other organic matter, and nitrate leaching, can significantly change soil N status before this time. The pre-sidedress nitrate test (PSNT) measures these potential changes. By complementing preplant soil testing with PSNT, growers can better predict yield response from N fertilizer, saving unnecessary fertilizer costs.
The PSNT is based on nitrate concentration in the top 12 inches of soil when corn is 6 to 12 inches tall (V6 growth stage). Under typical Colorado conditions, CSU researchers found the critical PSNT level is 15-ppm nitrate-N (NO3-N) in the top foot of soil at this growth stage. If the PSNT level is lower than 15 ppm NO3-N, sidedress N should be applied. If the PSNT level is higher than 15 ppm NO3-N, the probability of a yield response to additional N is very low (see Table 7). Although the PSNT was originally calibrated for non-manured fields in Colorado, the 15 ppm NO3-N should also be sufficient for fields with recent manure applications or legume crops. The test is most useful for predicting whether or not soil N is sufficient - not for making an N rate prediction. You must assess yield potential as well as soil nitrate levels to determine how much additional N is needed if the PSNT is below 15 ppm.
Table 7. The yield response
of corn to sidedress N application of 60 lbs/acre when PSNT was above
or below the critical NO3-N concentration at V6.
| (Sampling Depth: 0 - 12") |
Number of Observations
|
Yield response from sidedress N
|
Prediction accuracy
|
|
|
# of Sites
|
%
|
|
| Below critical level (15 ppm) |
35
|
19
|
54**
|
| Above critical level (15 ppm) |
21
|
0
|
100
|
| Total |
56
|
|
71
|
|
*Based on equal sampling intensity from both
furrow and shoulder positions |
|||
Proper soil sampling may be the most critical step in the PSNT procedure. To sample a field, take a minimum of 15 to 20 random soil core samples from a uniform soil area or 40-acre field. On surface irrigated fields, we recommend collecting equal numbers of soil samples from the furrow and shoulder of the bed and sampling depth of 12 inches (see Figure 2). Get the soil sample to a testing lab right away and tell the lab you are evaluating the sample for PSNT and need your results quickly. Using the PSNT will give you more confidence to evaluate your sidedress decision, and you may see savings in both fertilizer and sleep.
| Figure 2. Recommended sampling positions for PSNT testing of irrigated fields. |
Assessing plant N status
Measuring plant tissue or plant sap to evaluate N sufficiency has several
advantages and disadvantages. Benefits include ease of sampling, direct
feedback from the plant, and relatively low cost. However, tissue testing
is often inaccurate because of the many factors that impact plant N status
at the time of sampling, little calibration on the results, and poor sampling
technique. Although a tissue sample tells you what the N content of the
plant is at the time, it does not tell you what the soil reserve is.
Another method of evaluating N sufficiency during the growing season is to use the Minolta SPAD-502 chlorophyll meter. Although most research on the meter has been done on corn, it has also been researched on other crops including small grains, beans, and sugar beets. The portable chlorophyll meter provides instantaneous, nondestructive evaluation of plant nitrogen status, giving producers who have the ability to fertigate a tool to manage N more efficiently. The appropriate application of the chlorophyll meter is to compare the average greenness of corn leaves to a well-fertilized reference strip in the same field. When the average chlorophyll reading for the field drops below 95% of the reference strip, additional fertilizer is suggested.
Optimal SPAD reading times are between the 6-leaf and tassel (corn) stages. Corn hybrids differ in greenness, so do not compare readings of one hybrid to another to make fertilizer decisions. Systematic sampling technique and a sample size of at least 30 leaves per field is needed to accurately determine N response. Sampling the same leaf from each plant is important. For plants that have not tasseled, sample the uppermost fully expanded leaf with an exposed collar. After tassel, use the ear leaf. It is also very important to consistently measure greenness at the same location on the leaf. A sampling position halfway between the stalk and leaf tip will give a consistent reading.
by
Reagan Waskom
Extension Specialist
Water Resources
CSU Water Center
